Polishing sheet and polishing work method

ABSTRACT

The present invention provides a polishing sheet that can secure a flatness of a material to be polished and can improve a polishing efficiency. A polishing pad  1  has a polyurethane sheet  2  made of polyurethane resin. The polyurethane sheet  2  has large cells  3  with a generally triangular sectional configuration rounded along a thickness direction thereof. Polyurethane resin exists in the polyurethane sheet  2  in a partition wall manner and fine foams  4  are formed in the polyurethane resin. Fine particles  5  added during manufacture of the polyurethane sheet  2  exist inside some of the fine foams  4  and the fine particles  5  are separable from the fine foams. By separating off fine particles positioned at a polishing face P by dummy polishing or the like, fine foams which evenly reserve a polishing liquid containing abrasive particles are formed at the polishing face P.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polishing sheet and a polishing workmethod, and in particular relates to a polishing sheet having an elasticplastic foam sheet in which fine particles are contained and a polishingwork method using the polishing sheet.

2. Description of Related Art

Conventionally, a polishing sheet is used to perform flattening work ona material required for flatness at a high accuracy, such as an opticalmaterial such as a lens, a plane parallel plate, a reflecting mirror orthe like, a base plate for a hard disc, a silicon wafer, a liquidcrystal glass or the like. For example, as a polishing sheet used foraccurate flattening work, e.g. for the silicon wafer or the like, apolishing sheet of a nonwoven type obtained by drying a nonwoven sheetafter the nonwoven sheet is impregnated with a resin liquid is known(refer to JP05-8178A publication). However, since the polishing sheet ofthis type has flexibility, there are drawbacks in that a polishingefficiency is low and a roll-off, i.e. peripheral edge portions of amaterial to be polished are polished more than a central portionthereof, occurs easily. In order to improve the polishing efficiency,there has been disclosed a polishing sheet having an independent foamstructure obtained by hollow fine spherical bodies or the like in aplastic material with a high hardness (for example, refer toJP08-500622A publication) . However, because a polishing sheet of thistype has a high hardness, for example, when the sheet is used inpolishing work for an aluminum base plate for a hard disc, there is adrawback in that defects may occur at a surface of the aluminum baseplate.

On the other hand, a polishing sheet obtained by pasting a film with ahigh hardness or the like to an elastic plastic foam of a suede typewith a foam structure produced by wet film forming process has beendisclosed (for example, refer to JP10-249709A publication) . An elasticplastic foam is generally produced by the wet film forming process, andrelatively large cells extending in a direction approximatelyperpendicular to a polishing face are formed in the interior of theelastic plastic foam. Further, elastic plastic exists between adjacentcells in a manner of a partition wall, and relatively small fine foamswhose sizes have not been controlled are formed in the partition wall.

Further, a polishing sheet for a silicon wafer where fine particles ofbarium carbonate are contained in an elastic plastic foam as abrasiveparticles has been disclosed (for example, refer to JP2001-1270Apublication).

However, since the polishing sheet disclosed in the JP10-249709Apublication has an elastic plastic foam sheet as a base sheet, there areproblems about the polishing efficiency and the roll-off like that inthe JP05-8178A publication. Further, when the polishing sheet of theJP2001-1270 publication is used in polishing work for an aluminum baseplate, there is such a problem that defects may occur at a surface ofthe aluminum base plate by the fine particles contained in the polishingsheet. Accordingly, there is a need for development of a polishing sheetwhich can secure flatness of a material to be polished and has a highpolishing efficiency, and which does not cause defects at a surface ofthe material to be polished, such as an aluminum base plate or the like.

SUMMARY OF THE INVENTION

In view of the above circumstances, an object of the present inventionis to provide a polishing sheet which has an elastic plastic foam sheetas a polishing sheet and which can secure flatness of a material to bepolished and can improve a polishing efficiency, and a polishing workmethod using the polishing sheet.

In order to achieve the above object, a first aspect of the presentinvention is directed to a polishing sheet having an elastic plasticfoam sheet containing fine particles, wherein the elastic plastic foamsheet has a fine foam structure to be formed at a polishing face thereofby separating off the fine particles.

In the first aspect, since the elastic plastic foam sheet has a finefoam structure to be formed at a polishing face thereof by separatingoff the fine particles, fine foams which can reserve a polishing liquidcontaining abrasive particles are formed at the polishing face byseparating off the fine particles from the polishing face. Therefore,the abrasive particles contained in the polishing liquid are alwayssupplied to the fine forms formed at the polishing face duringpolishing, so that a polishing efficiency can be improved and a flatnessof a material to be polished can be secured.

In the first aspect, a continuously foamed body of polyurethane may beused for the elastic plastic foam sheet. Further, fine particles may becontained in the polishing face of the elastic plastic foam sheet to beseparable therefrom. In this case, it is preferable that the particlediameter of the fine particles is in the range of from 0.6 μm to 5 μm,since, when the diameter is less than 0.6 μm, sizes of the fine foamsare too small to allow the abrasive particles to be reserved in the finefoams, and when the diameter exceeds 5 μm, the strength of the elasticplastic foam sheet becomes lowered. If the fine particles are abrasiveparticles of at least one kind selected from ceric oxide, zirconia,alumina-zirconia, aluminum oxide, alumina ceramics, silicon dioxide,silicon carbide, diamond, ferric oxide, titanium oxide, manganesedioxide, calcium carbonate, and chromium oxide, the fine particles canbe easily separated off from the polishing face because these particleshave no coupling or bonding property with the elastic plastic foam. Inthis case, as a second aspect of the present invention, the elasticplastic foam sheet may have first fine foamed cells to be formed byseparating off the fine particles and second fine foamed cells that donot contain the fine particles at the polishing face.

Furthermore, a third aspect of the present invention is directed to apolishing work method for a material to be polished, which uses apolishing sheet which has an elastic plastic foam sheet in which fineparticles are contained and has a fine foam structure to be formed at apolishing face thereof by separating off the fine particles, comprisingthe steps of; attaching the polishing sheet to a surface plate of apolishing machine; and after all the fine particles are substantiallyseparated off from the polishing face, performing polishing work withthe polishing sheet to the material to be polished by using a polishingliquid containing abrasive particles.

According to the third aspect of the present invention, since all thefine particles are substantially separated off from the polishing faceprior to the polishing work, the same function and effects as the firstaspect can be achieved, and since the fine particles do not exist at thepolishing face of the material to be polished when the polishing work isperformed, occurrence of defects at the polishing face of the materialto be polished due to the fine particles can be prevented. At this time,the polishing machine for separating off all the fine particlessubstantially from the polishing face may be differed from a polishingmachine for conducting the polishing work, and dummy polishing may beperformed by the same polishing machine for separating off all the fineparticles substantially from the polishing face.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment where a polishing sheet according to the present inventionis applied to a polishing pad polishing for an aluminum base plate usedfor a hard disc will be explained with reference to the followingdrawings.

FIG. 1 is a sectional view illustratively showing a polishing pad of anembodiment to which the present invention is applicable;

FIG. 2 is a sectional view illustratively showing a state of thepolishing pad according to the embodiment where fine particles at apolishing face have been separated by dummy polishing;

FIG. 3 is a process diagram showing manufacturing process of a polishingpad;

FIG. 4 is a sectional view illustratively showing a polyurethane sheetbefore conducting buffing process;

FIG. 5 is a sectional view illustratively showing the polyurethane sheetafter conducting the buffing process;

FIG. 6 is a process diagram showing an outline of polishing workprocess;

FIG. 7 is a front view showing a schematic constitution of a polishingmachine;

FIG. 8 is a sectional view illustratively showing polishing work processfor an aluminum base plate;

FIG. 9 shows an electron microscopic photograph in the vicinity of apolishing face of a polishing pad of a comparative example 1 beforeconducting dummy polishing;

FIG. 10 shows an electron microscopic photograph in the vicinity of apolishing face of a polishing pad of a example 1 before conducting dummypolishing; and

FIG. 11 shows an electron microscopic photograph in the vicinity of apolishing face of a polishing pad of a example 1 after conducting dummypolishing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Polishing Pad

As shown in FIG. 1, a polishing pad 1 (a polishing sheet) has apolyurethane sheet 2 which is formed from polyurethane resin as anelastic (soft) plastic foam sheet whose one face is formed like nap. Thepolyurethane sheet 2 is formed with large cells (pores) 3 with agenerally triangular section, which are rounded along a direction of athickness thereof. The cell 3 is formed such that a size of a portion ofthe cell positioned at the side of a polishing face P opposed to a faceof a material to be polished is made smaller than that of a portion ofthe cell positioned at the side of an attaching face of the polishingpad to a polishing machine. Polyurethane resin exists in a manner of apartition wall between adjacent cells 3, and fine foams 4 dispersedgenerally evenly are formed in the polyurethane resin. These fine foams4 mutually connect 3-dimensionally in a network manner through finecommunication holes (not shown). Therefore, the polyurethane sheet 2 isconstituted as a continuously foamed body of polyurethane.

Fine particles 5 added at a time of manufacturing the polyurethane sheet2 are contained in some of the fine foams 4. The particle diameter ofthe fine particle 5 is set to a range of from 0.6 to 5 μm, and the finefoam 4 is set such that the size thereof is larger than the particlediameter of the fine particle 5. The fine foam 4 containing the fineparticle 5 generally has a space larger than the fine foam 4 which doesnot contain the fine particle 5. Further, the fine particles 5 which arepositioned at the polishing face P of the polyurethane sheet 2 andpositioned in the vicinity of an inner wall face of the cell 3 exist infine foams 4 which can be separated from the polishing face P and theinner wall face of the cell 3. Therefore, the fine particles 5 arecontained in the polyurethane sheet 2 and the polyurethane sheet 2 has afine foam structure to be formed at the side of the polishing face P byseparating off fine particles 5. Incidentally, the cells 3 and some ofthe fine foams 4 are opened at the polishing face P of the polyurethanesheet 2.

Further, the polishing pad 1 is provided, at an opposing face side (alower face side) to the polishing face P of the polyurethane sheet 2,with a film layer 7 made of polyethylene terephthalate (PET) which isutilized as a base material at a time of manufacturing the polyurethanesheet 2. A double adhesive tape 8 for attaching the polishing pad 1 to apolishing machine, which has a peeling paper (release paper) 9 on oneface side (a lowermost face side) thereof is adhered to a lower faceside of the film layer 7.

Manufacturing Method of Polishing Pad

As shown in FIG. 3, in order to manufacture a polishing pad 1, first,polyurethane resin, fine particles 5 and additives are blended inblending process. Polyurethane resin such as polyester base resin,polyether base resin or the like is used as the resin, and polyurethaneresin is dissolved in N,N-dimethyl-formaldehyde (hereinafter,abbreviated as DMF) of a solvent such that it occupies 30% in solution,so that polyurethane resin solution is obtained. As additives, pigmentsuch as carbon black or the like, hydrophilic activator for acceleratingfoaming, hydrophobic activator of film-forming stabilizer and the likecan be used in order to control the size or amount (number) of the finefoams 4.

As the fine particles 5, fine particles which have neither compatibilitywith polyurethane resin solution nor bonding property with apolyurethane resin and, for example, in case of fine particles forabrasive particles, fine particles of one kind or mixture of at leasttwo kinds selected from a group of ceric oxide, zirconia,alumina-zirconia, aluminum oxide, alumina ceramics, silicon dioxide,silicon carbide, diamond, ferric oxide, titanium oxide, manganesedioxide, calcium carbonate, and chromium oxide can be used. When theamount of addition of the fine particles 5 to the polyurethane resin isless than 1% by weight, change of the fine foam 4 is reduced (the sizeof the fine foam 4 is small) so that it becomes hard to exhibit effectsin polishing. On the contrary, when the amount of addition is more than100% by weight, because the polishing pad 1 itself is reduced instrength and separating-off property (from the polishing face P and theinner wall face of the cell 3) of the fine particles 5 is deteriorated,the amount is not preferable. The amount of addition of the fineparticles 5 is preferably in a range of from 1 to 100% by weight, morepreferably in a range of from 10 to 50% by weight.

Fine particles 5, additives such as pigment and the like, and DMF whichis a solvent are added in prepared polyurethane resin solution to bestirred and mixed sufficiently. After aggregated masses or the like areremoved by filtration, resin emulsion including fine particles 5 addedand mixed is prepared by carrying out defoaming under vacuum.

In the next film forming process, the prepared resin emulsion iscontinuously applied on a base member (the film layer 7) made of PETsuch that its thickness becomes substantially uniform by using a reversecoater, and the polyurethane resin is re-solidified by dipping the basemember in water to remove the DMF. When the DMF is removed from theresin emulsion in water, relatively large cells 3 are formed. At thistime, since the DMF is removed from the surface side of the resinemulsion applied rather than the base member side due to that the basemember hinders water from permeating thereinto, portions of the cells 3positioned at the side of the base member are formed so as to becomelarger than portions thereof at the surface side. Further, when the DMFis separated into water, fine foams 4 are formed in the polyurethaneresin in a continuous foaming manner. Incidentally, the fine foams 4 areformed in a partition wall of the polyurethane sheet 2 so as to bedistributed generally evenly.

Next, in buffing process, buffing (surface sanding) is performed to asurface of the polishing face P side of the sheet-like polyurethaneresin obtained, thereby removing a surface layer (so-called a skinlayer) of the polyurethane resin. FIG. 4 and FIG. 5 illustratively showa polyurethane sheet 2 before buffed and a polyurethane sheet 2 afterbuffed, respectively. By the buffing process, some of the cells 3 andthe fine foams 4 are opened to the polishing face P, so that thepolishing face P of the polyurethane sheet 2 obtains a nap property. Atthis time, the fine particles 5 positioned in the vicinity of thepolishing face P of the polyurethane sheet 2 are contained in the finefoams 4 such that they may be separated therefrom. Incidentally, thefilm layer 7 is omitted in FIGS. 4 and 5.

Here, the cells 3 are formed to be aligned in the obtained sheet-likepolyurethane resin in a thickness direction thereof and the polyurethaneresin exists in a partition wall shape for partitioning the cells 3. Inthe above-described film forming process, if control is carried out suchthat the sizes of the fine foams 4 are made large without changing theshape, the size of the cells 3 formed in the polyurethane sheet 2 andthe used amount of the polyurethane resin, the thickness of thepartition wall becomes large so that the area of the polishing face P(refer to FIG. 1) of the polyurethane sheet 2 becomes large.Accordingly, in the film forming process, by polishing efficiency duringpolishing work into consideration, control is carried out so as tosecure the thickness of the partition walls to become large, i.e., thearea of the polishing face P of the polyurethane sheet 2 to becomelarge.

Next, in laminating process, one face of a double adhesive tape 8 with apeeling paper 9 attached to the other face thereof is adhered to a faceopposite to the polishing face P of the base member (the film layer 7),and in the next cutting process, dies cutting is performed so as toobtain a desired shape, for example, a circle with a diameter of 640 mm.Subsequently, in inspecting process, an inspection is made forconfirming that there is neither stain nor adhesion of scraps, such asbuffs or the like, on a product, so that a polishing pad 1 is finished.

Polishing Work

Next, a polishing work method where polishing pads 1 are attached ormounted to a polishing machine for both surface polishing serving as apolishing machine and polishing work is conducted to an aluminum baseplate serving as a material to be polished will be explained.

As shown in FIG. 7, a polishing machine 20 used for polishing work isprovided with a base stand 23 in which a power source section, a controlsection, a motor for driving the polishing machine 20 and the like. Alower surface plate 22 for polishing a lower face of an aluminum baseplate that is a material to be polished is disposed above the base stand23. An upper surface plate 21 for polishing an upper face of thealuminum base plate, which has a center axis on an extension of a centeraxis of the lower surface plate 22, is disposed above the lower surfaceplate 22. Polishing pads 1 are respectively attached to an upper face ofthe lower surface plate 22 and a lower face of the upper surface plate21. The upper surface plate 21 and the lower surface plate 22 aresupported so as to be respectively rotatable in opposite directions(arrows in FIG. 7), and a rotational driving force for the upper surfaceplate 21 is conveyed from the motor inside the base stand 23 via avertical transmission shaft (not shown) housed in a post 30, ahorizontal support shaft 31 and a drive shaft 24, while a rotationaldriving force for the lower surface plate 22 is conveyed from the motorvia gears (not shown) and the like. Further, a surface plate verticalmoving cylinder 25 which can move the upper surface plate 21 verticallyis fixed to the horizontal support shaft 31. Furthermore, the driveshaft 24 extends through a supply disc 26 which has a plurality ofpolishing liquid supplying tubes 38 to supply the polishing liquid tothe upper surface plate 21 to be fixed to the upper surface plate 21. Aworking pressure adjusting cylinder 27 for adjusting a pressure whichpressurizes the upper surface plate 21 toward the lower surface plate 22during polishing work is disposed on the supply disc 26.

Further, one end of a liquid feeding pipe 37 for feeding the polishingliquid is connected to the supply disc 26, and another end thereof isconnected to a liquid feeding pump 36 for feeding the polishing liquid.The liquid feeding pump 36 is connected to a polishing liquid tank (notshown) which reserves the polishing liquid.

Polishing Pad Attaching Process

As shown in FIG. 6, in attaching process, first, the polishing pads 2are attached to the polishing machine 20. That is, the upper surfaceplate 21 is moved upwardly by the surface plate vertical moving cylinder21 to be separated from the lower surface plate 22, so that the peelingpaper 9 are peeled off from the polishing pads 1 and the polishing pads1 are pasted to the lower face of the upper surface plate 21 and theupper face of the lower surface plate 22. Incidentally, in this state,fine particles 5 are contained in the polishing faces P of the twopolishing pads 1 to be separable therefrom.

Dummy Polishing Process

In the next dummy polishing process, the upper surface plate 21 is moveddownward by the surface plate vertical moving cylinder 25, a dummymaterial to be polished which is equivalent to an aluminum base plate(an aluminum base plate different from a subject aluminum base plate forpolishing work) is sandwiched between the two upper and lower polishingpads 1, and then a dummy polishing for separating off fine particlesexisting at the polishing faces P of the polishing pads 1 and existingin the vicinity of inner wall faces of the cells 3 is conducted underthe conditions such that a rotational speed of a motor, an appliedpressure and the like are substantially equal to those in case of actualpolishing of an aluminum base plate. As shown in FIG. 2, in the dummypolishing process, all the fine particles positioned at the polishingfaces P of the two upper and lower polishing pads 1 and in the vicinityof the inner wall faces of the cells 3 are separated off from the finefoams 4 by pressing force from the working pressure adjusting cylinder27 and deformation of the partition walls due to rotations of the uppersurface plate 21 and the lower surface plate 22, and new fine openingsof the fine foams 4 which can reserve and allow the polishing liquid tomove are formed in addition of the openings of the fine foams 4 formedin the buffing process. As described above, in the film forming processfor the polishing pad 1, since the fine foams 4 are formed so as to bedistributed uniformly in the partition walls of the polyurethane sheet2, a state that the fine foams 4 are generally uniformly distributed atthe polishing face P can be obtained in this dummy polishing process.

Polishing Work Process

Next, in polishing work process, the dummy material to be polished isreplaced by a subject aluminum base plate for polishing work, and thenboth surfaces of the aluminum base plate are subjected to polishingwork. That is, as shown in FIG. 8, an aluminum base plate 40 issandwiched by two polishing pads 1 rotated together with the uppersurface plate 21 and the lower surface plate 22 due to pressure from theworking pressure adjusting cylinder 27, and a slurry-like polishingliquid 35 including abrasive particles is supplied between the polishingpads 1 and the aluminum base plate 40. The abrasive particles containedin the polishing liquid 35 enter into a space between the polishing pads1 and the aluminum base plate 40 so that both the surfaces (surfaces tobe polished) of the aluminum base plate 40 are subjected to polishingwork.

As described above, in the polishing pad 1 of this embodiment, the finefoams 4 are approximately uniformly distributed at the polishing face P,and the polishing liquid containing the abrasive particles is reservedinside the fine foams 4 and the cells 3, and it is movable through thecommunication holes formed in the network manner. The spaces (pores) ofthe fine foams 4 obtained by separating off the fine particles 5 in thedummy polishing process are larger than those of the fine foams 4 inwhich no fine particles exist originally. Accordingly, the former allowsthe abrasive particles to move in a higher degree of freedom than thelatter, and the number of the spaces of the fine foam 4 can becontrolled to increase by adjusting the addition amount of the fineparticles 5 in the blending process. For this reason, in the polishingprocess, the polishing work is conducted to the aluminum base plate 40such that the abrasive particles contained in the polishing liquid canmove between the fine foams 4 and the cells 3 through the communicationholes and that they can be generally uniformly and sufficiently suppliedto the faces to be polished of the aluminum base plate 40. Therefore,the polishing work can be conducted without causing polishing unevennesswhile flatness is being secured to the surface of the aluminum baseplate 40, and the polishing work is conducted approximately evenly toboth a peripheral portion and a central portion of the aluminum baseplate 40, thereby inferior work due to the roll-off can be reduced.

Further, in the polishing pad 1 of the present embodiment, it ispossible to secure the area of the polishing face P of the polyurethanesheet 2 to be larger by controlling the thickness of the partition wallsto become large. Therefore, since a contacting area of the polishingface P and the aluminum base plate 40 is increased by thus increasingthe area of the polishing face P of the polishing pad 1, it is madepossible to improve polishing efficiency.

Further, in the polishing work using the polishing pad 1 of thisembodiment, since all the fine particles 5 are substantially removed(separated off) from the polishing face P in the dummy polishingprocess, no fine particles 5 exist at the polishing face P in thepolishing work process. Accordingly, occurrence of the defects due tothe polishing work to the surface of the aluminum base plate 40 can beprevented.

In general, for polishing work to the aluminum base plate 40, a particlediameter of abrasive particles used is in a range of from 0.1 to 1 μm orso in primary polishing and it is in a range of from 0.001 to 0.1 μm orso in secondary polishing. In this embodiment, since the fine foam 4 isformed such that its size is larger than the particle diameter of thefine particle 5, the fine foam 4 having a desired size can be easilyrealized by changing the particle diameter of the fine particles 5 thatare added when manufacturing the fine form 4. If the diameter of theparticle size 5 is smaller than 0.6 μm, the size of the fine foam 4becomes too small to reserve the abrasive particles, so that it becomesdifficult to enhance the polishing efficiency. On the contrary, if theparticle diameter is larger than 5 μm, the strength of the polishing pad1 and the separating-off property of the fine particles 5 are influencedbecause the thickness of the partition walls of the polyurethane resinis in a range of from 5 to 10 μm or like at thin portions thereof. Dueto these reasons, for polishing, the particle diameter of the fineparticle 5 is preferably from 0.6 μm to 5 μm, and more preferably from 1μm to 3 μm. By controlling the sizes of the fine foams 4 and thecommunication holes in the film forming process, the movement of thepolishing liquid including the abrasive particles is made easy insidethe polishing pad 1, between the cells 3 and between the fine foams 4.Therefore, not only in the secondary polishing but also in the primarypolishing, the polishing efficiency can be improved in cooperation withsupplying of the polishing liquid from the inside of the polishing pad 1and the cells 3 to the fine foams 4.

Incidentally, in this embodiment, the polyurethane resin sheet has beenexemplified as the elastic plastic foam sheet, but this invention is notlimited to this example. For example, resin which forms continuous foamssuch as polyester resin may be used. When the polyester resin is used,continuous foams may be easily formed according to the wet film formingprocess. Further, in this embodiment, the wet film forming process hasbeen exemplified as the process for manufacturing the polyurethanesheet, dry film forming process maybe used. At this time, for example,hydrophilic activator or the like for promoting foaming may be added toform continuous foam body.

Further, in this embodiment, the coating of the resin emulsion to thePET-made base material by the reverse coater has been exemplified,however, for example, a roll coater or a knife coater may be used. Anycoater may be used as long as it can allow uniform coating to the basematerial. Further, for example, a nonwoven sheet, a woven sheet or thelike may be listed as a usable base material other than the PET.

Furthermore, in this embodiment, the example that the polyurethane sheet2 is formed on the base material made of the PET has been shown, but itis possible to obtain only the polyurethane sheet 2 by peeling off thebase material that is solidified in a wet type manner after applying theresin emulsion to the PET, and the polyurethane sheet 2 may be directlyattached to the surface plate of the polishing machine. In this case,other base material may be adhered to the polyurethane sheet 2, and anonwoven sheet or a woven sheet made of synthetic fibers may be used asthe base material other than the film made of the PET.

Further, in this embodiment, the polishing machine 20 which polishesboth surfaces of the material to be polished has been exemplified as thepolishing machine, but the present invention may, of course, be appliedto a polishing machine which polishes one surface of a material, and apolishing machine which polishes a resin-made lens, a spherical glass orthe like. Furthermore, in this embodiment, the example that thepolishing pads 1 are attached to the polishing machine 20, and the fineparticles at the polishing face are separated off by the dummy polishinghas been shown, but the polishing pad 1 may be attached to the polishingmachine 20 after the fine particles 5 of the polishing pad 1 have beenseparated off in advance by using an exclusive polishing machine fordummy polishing, an exclusive machine for separating off fine particles(for example, which may be a machine for carrying out the separating-offby utilizing vibrations) or the like. By employing such an exclusivemachine, the fine particles 5 are almost entirely prevented fromentering into the polishing machine 20. In the above-describedembodiment, the example that fine particles are separated off from thepolishing face P of the polishing pad 1 under the same conditions asthose in the case that the aluminum base plate is polished has beenshown, however, it is unnecessary to make the conditions in the dummypolishing process equal to those in the polishing work process since itis sufficient in this invention that once the fine particles areseparated off from the polishing face P of the polishing pad 1.

Next, examples of the polishing pad 1 manufactured according to theabove embodiment will be explained. Incidentally, comparative examplesmanufactured for comparison will also be described.

EXAMPLE 1

As shown in the following Table 1, in Example 1, white fused aluminaparticles having an average particle diameter of 2.0 μm were used as thefine particles 5, and polyester-diphenylmethane diisocyanate (MDI) basepolyurethane resin was uses as the polyurethane resin. Resin emulsionwas prepared by adding and blending white fused alumina of 40 weightparts, a DMF dispersing liquid of 40 weight parts including carbon blackof 30% as pigment, and hydrophilic activator of 2 weight parts,hydrophobic activator of 2 weight parts and DMF of 52 weight parts, toDMF solution of 100 weight parts including polyurethane resin of 30%.The polishing pad 1 was manufactured by using this resin emulsion. TABLE1 ADDITION AMOUNT (WEIGHT PART) TO DMF SOLUTION OF 100 PARTICLE WEIGHTPARTS INCLUDING DIAMETER POLYURETHANE RESIN OF (μm) 30% EXAMPLE 1 2.0 10EXAMPLE 2 0.6 10 EXAMPLE 3 5.0 10 EXAMPLE 4 2.0 3 EXAMPLE 5 2.0 10EXAMPLE 6 2.0 15 COMPARATTVE NON- 0 EXAMPLE 1 ADDITION COMPARATIVE 2.030 EXAMPLE 2

EXAMPLE 2 To EXAMPLE 3

As shown in Table 1, in Examples 2 and 3, the same conditions as thosein Example 1 were employed except for a difference in a particlediameter of the white fused alumina particles. In Example 2, theparticle diameter was 0.6 μm and in Example 3, it was 5.0 μm.

EXAMPLE 4 To EXAMPLE 6

As shown in Table 1, in Examples 4 to 6, the same conditions as those inExample 1 were employed except for a difference in an addition amount ofthe white fused alumina. In Example 4, the addition amount of the whitefused alumina was 3 weight parts, in Example 5, it was 10 weight parts,and in Example 6, it was 15 weight parts.

COMPARATIVE EXAMPLE 1

As shown in Table 1, in Comparative Example 1, the same conditions asthose in Example 1 were employed except for non-addition (0 weightparts) of the white fused alumina.

COMPARATIVE EXAMPLE 2

As shown in Table 1, in Comparative Example 2, the same conditions asthose in Example 1 were employed except for an addition amount of thewhite fused alumina of 30 weight parts. However, a polishing pad couldnot be manufactured because film forming was not achieved.

Dummy Polishing And Polishing Work

By using the polishing pads of Examples and Comparative Examples, dummypolishing and polishing work were conducted under the followingconditions:

Used Polishing Machine: 9B-5P POLISHING MACHINE manufactured by SPEEDFIRM INC.

Polishing Speed (rpm): 25r/m

Working Pressure: 100 g/cm²

Slurry (Polishing Liquid): DL3471 produced by FIJIMI CORPORATED (averageparticle diameter: 0.8 μm; Mixed liquid of DL3471:water=1:3 was used.)

Slurry Supplying Amount: 100 cc/min

Kind of Material to be Polished: aluminum base plate for 95 mmφ harddisc

Polishing Time: one minute; three minutes

Evaluation

Next, the polishing pads of Examples and Comparative Examples wereevaluated on the basis of improvement degree in waviness calculated froma polishing rate to an aluminum base plate and waviness thereof.Further, regarding polished aluminum base plates, appearance evaluationabout presence/absence of defect occurrence to surfaces of the aluminumbase plates was made according to visual inspection. Furthermore,section structures of the polyurethane sheets of Examples andComparative Examples in the vicinity of the polishing faces P thereofwere confirmed by microscopic photographs.

Polishing Rate

The polishing rate is one of numerical values indicating a polishingefficiency and it represents a polishing amount per one minute with athickness. Weight reduction of the aluminum base plate was calculatedfrom measured weight before the polishing work and measured weight afterthe polishing work, and then the polishing rate was calculated from thepolished area and the specific gravity of the aluminum base plate.

Improvement Degree In Waviness

Waviness is one of measurement items for evaluating a surface accuracy(flatness) to a disc base plate, a silicon wafer or the like, it isexpressed with angstrom unit (A) to a waviness amount (Wa) per unit areaof a surface image which is observed by an optical non-contactingsurface roughness tester. Evaluation was made by using the “Optiflat” asa test and evaluation machine. In particular, in a disc base plate usedfor a fixed magnetic disc (a hard disc) device used in combination witha floating type magnetic head, since floating property of the magnetichead deteriorates according to an increase of this waviness, it isimportant to suppress the waviness as small as possible during thepolishing work. As the measured numerical value becomes lower, thewaviness becomes small, accordingly, which means that a more flat faceis realized. An improvement degree in waviness is a difference in awaviness amount (Wa) between the aluminum base plate before and afterthe polishing work, and an increase in the improvement degree means areduction in the waviness amount, which indicates that a flatness of thealuminum base plate is secured.

Regarding the polishing pads 1 of Example 1 to Example 3 and thepolishing pad of Comparative Example 1, the results obtained byconducting the polishing work tests about a polishing time of one minuteand evaluating the polishing rates, the improvement degrees in wavinessand the appearances of these polishing pads are shown in Table 2.Further, regarding the polishing pads 1 of Example 1 to Example 6 andthe polishing pad of Comparative Example 1, the results obtained byconducting polishing work tests about a polishing time of three minutesand evaluating the polishing rates, the improvement degrees in wavinessand the appearances of these polishing pads are shown in Table 3. TABLE2 POLISHING IMPROVEMENT RATE DEGREE IN (μ/min) WAVINESS(Å) DEFECTSEXAMPLE 1 0.8 2.21 Non EXAMPLE 2 0.8 1.90 Non EXAMPLE 3 0.9 2.04 NonCOMPARATIVE 0.8 1.85 Non EXAMPLE 1

As shown in Table 2, regarding the polishing rate, all of the polishingpads 1 of Example 1 to Example 3 where the white fused alumina was addedand the polishing pad of Comparative Example 1 where no white fusedalumina was added were stable and there was found no difference amongthem. Further, regarding the appearance, no defects were found at thesurfaces of the polished aluminum base plates. The polishing pads 1 ofExample 1 to Example 3 were improved largely regarding the waviness ascompared with the polishing pad of Comparative Example 1. It wasconfirmed that the improvement degree of Example 1 where the white fusedalumina particles with the particle diameter of 2.0 μm were added washigh among them.

As shown in Table 3, the improvement degree in waviness is improved byextending the polishing time. However, the polishing pads of Examplesare higher in improvement degree than the polishing pad of ComparativeExample 1 even if the polishing time is extended. Furthermore, regardingthe addition amount of the white fused alumina, it was confirmed thatthe improvement degree in waviness of Example 5 including the whitefused alumina of 10% by weight was high. TABLE 3 POLISHING IMPROVEMENTRATE DEGREE IN (μ/min) WAVINESS(Å) DEFECTS EXAMPLE 1 0.7 3.75 NonEXAMPLE 2 0.7 3.05 Non EXAMPLE 3 0.8 3.33 Non EXAMPLE 4 0.7 3.20 NonEXAMPLE 5 0.7 3.75 Non EXAMPLE 6 0.6 3.35 Non COMPARATIVE 0.7 2.90 NonEXAMPLE 1 COMPARATIVE (Film Forming Impossible) EXAMPLE 2

FIGS. 9 and 10 show electron microscopic photographs of portions in thevicinity of the polishing faces of the polishing pads of ComparativeExample 1 and Example 1 before the dummy polishing is conducted. Asapparent from FIG. 10, it is found that the alumina particles with theparticle diameter of 2 μm are dispersed evenly in the polishing pad ofExample 1. Further, FIG. 11 shows an electron microscopic photograph ofa portion in the vicinity of a polishing face of the polishing pad 1 ofExample 1 after the dummy polishing was conducted. Almost all thealumina particles at the polishing face and the inner wall faces of thecells were separated and only a slight amount of the particles confinedin the partition walls remain in the polishing pad. Thus, it is possibleto change the sizes and the number of fine foams in the partition wallsby addition of the alumina particles.

1. A polishing work method for a material to be polished, which uses apolishing sheet which has an elastic plastic foam sheet in which fineparticles are contained and has a fine foam structure to be formed at apolishing face thereof by separating off the fine particles, comprisingthe steps of; attaching the polishing sheet to a surface plate of apolishing machine; and after all the fine particles are substantiallyseparated off from the polishing face, performing polishing work withthe polishing sheet to the material to be polished by using a polishingliquid containing abrasive particles.
 2. A polishing work methodaccording to claim 1, wherein all the fine particles are substantiallyseparated off from the polishing face due to dummy polishing by thepolishing machine.